The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for dimming control with a system controller. Merely by way of example, the invention has been applied to light-emitting-diode (LED) driving systems. But it would be recognized that the invention has a much broader range of applicability.
Light emitting diodes (LEDs) have been widely used in various lighting applications because LEDs have significant advantages, such as high efficiency and long lifetime, over other lighting sources (e.g., incandescent lamps). LED lighting systems often use a conventional light dimmer that includes a Triode for Alternating Current (TRIAC) to adjust the brightness of LEDs. Such a conventional light dimmer is usually designed to drive pure resistive loads (e.g., incandescent lamps), and yet may not function properly when connected to capacitive loads, such as LEDs and/or associated circuits.
When the conventional light dimmer starts conduction, internal inductance of the light dimmer and the capacitive loads may cause low frequency oscillation. Hence, the Alternate Current (AC) waveforms of the conventional light dimmer often becomes unstable and/or distorted, resulting in flickering, undesirable audible noise, and/or even damages to other system components. FIG. 1 shows simplified signal waveforms of a conventional light dimmer that is connected to capacitive loads. The waveform 104 represents a voltage signal generated from a conventional light dimmer, and the waveform 102 represents a rectified signal generated from the voltage signal.
Some measures can be taken to solve the above problems in using a conventional light dimmer with capacitive loads such as LEDs and/or associated circuits. For example, a power resistor (e.g., with a resistance of several hundred Ohms) may be connected in series in an AC loop to dampen initial current surge when the light dimmer starts conduction.
FIG. 2 is a simplified diagram showing a conventional light dimmer system. The light dimmer system 200 includes a light dimmer 204, a rectifier 206, a capacitive load 208, and a power resistor 210. As shown in FIG. 2, the light dimmer 204 receives an AC input 202, and generates a signal 212 which is rectified by the rectifier 206. The rectifier 206 outputs a signal 214 to the capacitor load 208. The power resistor 210 serves to dampen the initial current surge when the light dimmer 204 starts conduction.
FIG. 3 shows simplified conventional signal waveforms of the light dimmer system 200. As shown in FIGS. 2 and 3, the waveform 304 represents the signal 212, and the waveform 302 represents the rectified signal 214. As shown by the waveforms of FIG. 3 compared with the waveforms in FIG. 1, using the resistor 210 in the light dimmer system 200 can reduce low frequency oscillation, and in addition the rectified signal 214 does not show any significant distortion. But, for the light dimmer system 200, a current would flow through the resistor 210 even under normal working conditions, causing excessive heating of resistor and other system components. Such heating often leads to low efficiency and high energy consumption.
Some conventional techniques would short the power resistor through peripheral circuits when the AC input is stabilized after a light dimmer conducts for a predetermined period of time. FIG. 4 is a simplified diagram showing a conventional system for dimming control. The system 400 includes an AC input 404, a light dimmer 402, a damping control circuit 406, a power train 408 and one or more LEDs 488. The damping control circuit 406 includes a power transistor 460, a capacitor 462, and resistors 472, 474, 476, 478 and 480. For example, the resistor 480 is the same as the resistor 210. In another example, the power transistor 460 is a N-type MOS switch.
As shown in FIG. 4, when the light dimmer 402 (e.g., a TRIAC) is turned off, the transistor 460 is turned off by the voltage divider including the resistors 472, 474 and 476. When the TRIAC light dimmer 402 begins conduction, a delay circuit including the resistors 472 and 474 and the capacitor 462 causes the transistor 460 to remain off, while the resistor 480 dampens an initial surge current. After a delay, the transistor 460 is turned on again, and hence the resistor 480 is shorted.
Though the system 400 often has a better efficiency compared with the system 200, the system 400 still suffers from significant deficiencies. For example, the system 400 usually needs many peripheral devices in order to operate properly. In addition, the cost of the system 400 is often very high.
Hence it is highly desirable to improve the techniques of dimming control.